The present invention relates to forming circuit patterns in semiconductor wafers using electron beam lithography and, more particularly, to an apparatus and method for mounting a ceramic ferrite pole piece in an electromagnetic lens of an electron beam system used in such lithography.
Pole pieces must be located accurately within an electromagnetic lens. Ferrite is a magnetic ceramic, and a pole piece made of ferrite is the focusing part of the lens. It is critical that the pole piece be centered in the lens so that an electromagnetic field which focuses the electron beam is centered. In conventional electron beam systems, the centering of the ceramic ferrite pole piece is accomplished by inserting the pole piece into a metal centering plate having an opening whose diameter is larger by only a very small tolerance than the outer diameter of the pole piece. Ferrite pole pieces are fragile, and the close tolerance between the pole piece and the centering plate subjects the pole piece to chipping and even breaking during assembly and disassembly. The pole pieces must be removed sometimes for maintenance and then replaced. This increases the exposure of the pole piece to chipping and breaking. Although external chipping does not pose a serious problem in itself, it can lead to the breaking of a pole piece. The ferrite used for the pole pieces is expensive and has a long manufacturing lead time.